1. Field of the Invention.
This invention pertains to welding apparatus, and more particularly to portable apparatus suitable for welding sheet metal work pieces.
2. Description of the Prior Art.
It is well known to join two pieces of sheet steel or other metal by means of electrical spot welding and brazing. For example, the Miller Electric Company, Appleton, Wis., manufactures a spot welding machine having rigid tongs extending therefrom that serve as holders for the welding electrodes. Spot welders are especially useful in the motor vehicle repair industry for welding body replacement panels. However, prior spot welders involve various hazards and do not provide consistent results. The prior welders are of only limited application; they are too heavy, bulky, and difficult to manuever for use under an automobile. Also, prior welders used for automobile repair tend to burn the automobile primer and create a slag that must be ground off, thereby requiring wasteful cleanup time.
Sheet metal parts may be brazed or welded together with oxygen-acetylene torches, but such torches are inconvenient to use in many locations on a motor vehicle. In addition, torches present a fire hazard because of the presence of vehicle upholstery and gasoline. They also are hazardous to the operator.
Wire welders are known, but they have only limited application, because holes must be drilled in the parts and a button or plug welding technique must be used. After welding, the remaining button of weld material must be ground off. Also, the parts must be clean and free of rust. Wire welders have a high potential for burning upholstery, painted surfaces, and the operator with the very hot sparks produced. The sparks are especially hazardous due to the presence of various volatile solvents found in automobile repair shops.
Prior spot welders are further handicapped by the lack of suitable cooling to the electrodes, although cooling means are known. For example, U.S. Pat. No. 1,534,688 shows a gas cooled electric welding device. The device includes a flexible coupling having a tube portion joined to a holder in which is mounted an electrode. The flexible coupling is specifically designed to conduct air and thus adds undesirable expense to the welding device. Moreover, projecting coolant directly at the electrode, as in the U.S. Pat. No. 1,534,688, may overcool the electrode and thus increase the welding time.
Another disadvantage of prior spot welders is the difficulty in accurately controlling the electrode clamping pressure on the work parts on a repeated basis. If the clamping pressure is too great, the electrodes will deform. Also, heat will be conducted too rapidly from the electrodes and thus increase the welding time required and produce poor welds. Too little clamping force results in poor electrical conductivity, which produces poor welding results.
Thus, a need exists for a versatile spot welder that accurately and precisely controls the clamping pressure and temperature variables associated with the welding operation.